IB/mlx5: Don't expose debugfs entries for RRoCE general parameters if not supported

debugfs entries for RRoCE general CC parameters must be exposed only when
they are supported, otherwise when accessing them there may be a syndrome
error in kernel log, for example:

$ cat /sys/kernel/debug/mlx5/0000:08:00.1/cc_params/rtt_resp_dscp
cat: '/sys/kernel/debug/mlx5/0000:08:00.1/cc_params/rtt_resp_dscp': Invalid argument
$ dmesg
 mlx5_core 0000:08:00.1: mlx5_cmd_out_err:805:(pid 1253): QUERY_CONG_PARAMS(0x824) op_mod(0x0) failed, status bad parameter(0x3), syndrome (0x325a82), err(-22)

Fixes: 66fb1d5d

 ("IB/mlx5: Extend debug control for CC parameters")
Reviewed-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Link: https://lore.kernel.org/r/e7ade70bad52b7468bdb1de4d41d5fad70c8b71c.1706433934.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>